More evidence linking chicken litter and toxic arsenic

(11.01.2007)

Microorganisms of the genus Clostridium in chicken litter release arsenic from the feed additive roxarsone much faster and more effectively than previously assumed.

Background:

Roxarsone, or 3-nitro-4-hydroxybenzene arsonic acid, is routinely used by most U.S. broiler-chicken farms as a feed additive to prevent disease and stimulate growth (see EVISA News:
The use of arsenic in "poultry industry"). Chickens primarily excrete roxarsone unchanged into their waste, which is typically applied as a fertilizer on the surrounding farmlands.

The addition of roxarsone to chicken feed has stirred up debate ever since it was revealed that this organoarsenic compound can be transformed into more toxic inorganic arsenic. The toxic arsenic species can then be be found both partly in the meat (see EVISA News) but mainly in the chicken excreta that is brought onto farmland and therefore might also reach ground water.

Previous research had shown that inorganic arsenic is slowly formed from roxarsone after litter composting or field application, but the responsible microorganisms or processes were not identified.

The new study:New research published yesterday on ES&T’s Research ASAP website (DOI: 10.1021/es061802i) identifies the microorganisms that are responsible for the conversion and shows that carcinogenic arsenic (As) is formed faster and more effectively than previously thought.

John Stolz and his colleagues at Duquesne University now show that roxarsone is rapidly transformed to 3-amino-4-hydroxybenzene arsonic acid and inorganic As(V) under anaerobic conditions by the bacteria of the Clostridium species.

“We see As(V) emerge in less than 10 days, which is much faster than previously observed,” says Stolz, who emphasizes that Clostridium species are indeed the dominant bacteria in chicken cecum and litter. In this way, inorganic arsenic could already be generated during manure storage, he says. The process is so efficient because the microbes gain energy from the roxarsone transformation and couple it to growth, presumably through an anaerobic respiration mechanism in which roxarsone serves as the terminal electron acceptor, Stolz explains.

Comments:“This is really elegant work that amplifies previous evidence on the environmental release of inorganic arsenic from roxarsone,” says Ellen Silbergeld of the School of Public Health at Johns Hopkins University. “I am concerned about the potential contamination of groundwater with inorganic arsenic in my local region [in Maryland], because we have a lot of poultry farms and most people here get their drinking water from groundwater,” she says.

On a broader scale, Silbergeld feels that the use of roxarsone is impeding the opportunities for alternative uses of poultry waste. “One of my students found inorganic arsenic in pelletized chicken waste that is sold as a garden fertilizer, and in this way people could get exposed to the arsenic through dust—[it’s] probably not such a good idea to use chicken waste in that way,” she adds. The coauthor of the new ES&T research, environmental chemist Partha Basu of Duquesne University, points out that a recent study found elevated arsenic levels in house dust near chicken farms (Environ. Forensics 2005, 6, 83–89).

“In my view, the real threat of arsenic mineralization from roxarsone is the buildup of inorganic arsenic in agricultural soils onto which poultry litter is applied as manure,” says Andrew Meharg of the University of Aberdeen (U.K.), an expert in the biogeochemistry of arsenic. As(V) is relatively immobile in aerobic soils and, therefore, does not pose much of a risk there at slightly elevated soil concentrations, Meharg says. However, if arsenic levels build up, a possibility exists that it could transfer into food crops, he cautions. If the soils are used for anaerobic cultivation (e.g., rice), then the risk becomes larger because of transformation of As(V) into the more mobile As(III), he adds.

“The fact that roxarsone is still routinely used here [in the U.S.] and that they ignore that something toxic is getting into the environment is causing me a problem,” says Stolz. He points to a recent report by the Institute of Agriculture and Trade Policy, a nonprofit research and advocacy organization, which notes that the uptake of part of the roxarsone into the chicken body leads to elevated arsenic concentrations in the chicken meat sold in U.S. supermarkets. “The insidious thing about arsenic is the fact that it takes people decades to develop symptoms to chronic exposure,” Stolz says. “Some big U.S. companies raise chickens without using roxarsone and appear to manage,” he adds.